The long-term objective is to understand how and why mammalian cells sense, or on the other hand escape from senescence and become immortal. The specific aims are: 1) To identify murine genes whose expression changes during senescence and/or during immortalization of mouse embryo fibroblast in cell culture. 2) To discover why these changes in gene expression have occurred and what is responsible for them. Two families of genes, one encoding MRP/proliferin and a second encoding retroviral-related sequences, show a correlation between increase expression and the acquisition of immortality by mouse embryo fibroblast inculture. They thus provide the necessary handle to identify at what level (e.g. splicing, mRNA stability) changes in gene regulation are occurring. It will be of particular interest to ascertain the role of the retroviral sequences in immortazation. The macromolecular species (protein, possibly RNA) responsible for the changes in gene expression will be isolated and characterized. It is important of understand how this (these) component(s) contribute(s) to immortalization in particular and regulation of gene expression in general. Recombinant DNA technology will be used of generate cDNA clones of MRNA from mortal and immortal cells. The libraries of molecular clones will be screened to identify specific clones corresponding to genes whose expression, as assessed by the presence or absence of the mRNA in the cytoplasm correlates with immortalization. Clones of mRNAs that disappear, or are reduced in abundance, when cells become immortal will be the maker focus, These will be candidates for a positive regulator of mortality. This project relates to all diseases in which changes in gene expression occur, notably aging and cancer. Because studies on regulation of gene expression in mammalian cells are involved, the work may impact on other areas also.